The production of heavy oil and bitumen from a hydrocarbon reservoir is challenging. One of the main reasons for the difficulty is the viscosity of the heavy oil or bitumen in the reservoir. At reservoir temperature, the initial viscosity of the oil is often greater than one million centipoises, which is difficult to produce if not mobilized using external heat. Therefore, the removal of oil from the reservoir is typically achieved by introducing sufficient energy into the reservoir to heat the reservoir, such that the viscosity of the oil is reduced sufficiently to facilitate oil production.
Currently the preferred method of introducing energy into the reservoir is steam injection. The heat from the steam reduces the viscosity of the fluid, allowing it to flow toward production wells. The steam also provides voidage replacement to maintain the pressure in the reservoir. Cyclic Steam Stimulation (CSS), steam drive, and Steam Assisted Gravity Drainage (SAGD) all use steam for heating and maintaining pressure in the reservoir.
In a typical cyclic steam production, as shown in FIG. 1, steam is injected into the reservoir and then allowed to “soak” as it transfers heat to the reservoir. This period is followed by a production period. When the oil production rate again diminishes, steam is again injected into the reservoir and the cycle is repeated. The steam injection during CSS provides heat and pressure support to enable production of the heavy oil or bitumen.
Although steam assisted oil production has proven to be quite valuable, it is not without drawbacks. Steam based methods for stimulating reservoirs containing heavy oil or bitumen use significant amounts of energy and water, most notably the energy to generate the steam in high temperature and transfer the steam into the reservoirs. Moreover, the steam injected into the reservoirs will eventually condense to water and is retrieved. Thus, it will require additional facilities and energy to treat the water before it can be recycled or exhausted. Finally, the availability of water on site may be a limiting factor in certain locations. Thus, other methods of transferring heat to heavy oils are of interest in the art.
For example, using microwave or radio frequency radiation to heat the oil reservoir and mobilize the oil have long been known in the art. U.S. Pat. No. 3,133,592 disclosed an apparatus for treating a subsurface petroleum reservoir by using a series of vertically spaced microwave energy generating units and means for generating and directing microwave energy into the reservoir to heat and mobilize the oil contained therein.
However, microwave radiation has limited penetration in oil sands, for instance at 2.45 GHz radio frequency and for rich Athabasca oil sands, which have an electrical conductivity of 0.002 mhos/meter, the 1/e or 64% penetration depth of electromagnetic heating energy may be only 9 inches. Thus, radio frequencies between about 0.001 and 30 MHz may be preferred.
U.S. Pat. No. 5,082,054 disclosed an in situ method for partially refining and extracting petroleum from a reservoir by irradiating the reservoir with electromagnetic energy, mainly in the microwave region, to heat and partially crack the hydrocarbons in the reservoir. However, to effect in situ upgrading the energy supplied has to be large enough to increase the temperature within the reservoir sufficient to trigger the cracking process. Thus this process is energetically quite expensive.
U.S. Pat. No. 6,189,611 discloses a method of producing a pool of subterranean fluid by radiating and modulating electromagnetic energy. However, U.S. Pat. No. 6,189,611 recites more or less continuous application of very large amounts of RF energy, sufficient to vaporize a portion of the hydrocarbon and propagate a material displacement bank away from the applicator well. It does not, however, contemplate a more limited usage of RF that is combined with a soak period, nor a limited RF combined with cyclic steam stimulation.
U.S. Pat. No. 7,091,460 discloses a method of automatically detecting and adjusting the radio waves used to heat hydrocarbon formations. Specifically, the patent measures an effective load impedance and compares that with an output impedance of a signal generating unit so as to match the former with the latter. Thus, U.S. Pat. No. 7,091,460 achieves an electrical load match while subjecting the transmission line to reflected energy circulation, e.g. a high voltage standing wave ratio. The resulting high power factor may cause transmission inefficiency so that the megawatt power levels of real world wells become difficult or impossible to attain. Further, the method is complicated and contributes to operating costs.
US2009173488 discloses a system for recovering oil from an oil shale deposit using a microwave generation system and a sheath to shield the antenna from harmful exposure to the corrosive oil components. The sheath, however, may not be necessary, as our work indicates that corrosion is not a problem.
Thus, what is needed in the art are better and more cost effective ways of using RF radiation to provide heat to a reservoir for enhanced oil recovery.